That and check the directions when you buy light sets. Some are NOT compatible with timers. I bought a timer and had same happen. Then when I read backing of light fixture it is NOT compatible with timer and says so! Who knew a plant light wouldn't be timer compatible. Ah to sooner or later replace it with regular hood. Fish know it is feeding time when I turn it on in morning and off at night! LOL

But sometimes there is a compatiblility issue which will cause it to stop working.

What kinda light wouldn't be compatible with a timer? For me that raises a instant red flag on the quality of the parts inside of it. I can understand a higher powered light fixture needed to be grounded, but honestly no idea why a timer wouldn't work with it.